Phosphate recovery from wastewater

ABSTRACT

Provided are methods for recovering phosphate from wastewater by treating the wastewater with calcium containing compounds to raise its pH to near neutral values in order to precipitate calcium phosphate compounds, such as brushite, from the wastewater.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 13/942,900, filed on Jul. 16, 2013, which is a continuation of U.S.patent application Ser. No. 12/775,230, filed on May 6, 2010, whichclaims priority to U.S. provisional patent application No. 61/215,534,filed on May 7, 2009, the entire contents of each of which are herebyincorporated by reference.

TECHNICAL FIELD

The present invention relates to a process and system for recoveringphosphate from the waste stream at sewage treatment plants.

BACKGROUND OF THE INVENTION

Phosphorus is both a plant and animal nutrient and an environmentalcontaminant in the modern world, implicated as a major source ofeutrophication of surface waters. Both urban and agricultural wastestreams contain phosphorus that entered the element cycle as a nutrientbut that is difficult to remove and recover in a recyclable form and,therefore, is more nuisance than nutrient. Sewage treatments plants areobliged to reduce phosphorus levels in discharge water to low levels,but typically do so by directing the phosphorus to the sewage sludge, orbiosolids, which are usually land applied. In doing so, sewage treatmentplants are often faced with nuisance formation of phosphate minerals,principally struvite, in pipes, heat exchangers, and tanks due to thehigh levels of phosphate produced during anaerobic digestion of thesolids. The biosolids have an unfavorably high phosphorus/nitrogenratio, so that if biosolids are land-applied to meet crop nitrogen needsthen the added phosphorus exceeds crop needs and will either accumulateto undesirable levels in the soil or be prone to runoff losses witherosion. Rock phosphate mineral resources for fertilizer production arenonrenewable and limited to another century at current rates of use.

The production of biosolids in sewage treatment plants employs anaerobicdigestion of a combination of two feedstocks: first, primary sludgeproduced by produced by settling and grit removal of the raw sewage, andsecond, waste activated sludge produced from the treated water bybiological nutrient removal using polyphosphate accumulating organismsto accumulate phosphorus in their biomass before wasting to beanaerobically digested as part of the biosolids.

A number of methods have been devised to recover phosphorus from sewagetreatment plants. There are basically two processes to recoverphosphorus from wastewater using crystallization reactions: thehydroxylapatite process and the magnesium ammonium phosphate process. Inthe hydroxylapatite process, a calcium source is added into thewastewater and phosphorus is recovered in the form of calcium phosphate(Hirasawa et al, 1981a, 1981b, 1981c). In the magnesium ammoniumphosphate process (Ohlinger et al., 1998; Durrant et al., 1999;Shimamura et al., 2003; Yoshino et al, 2003), a magnesium source isadded (often as magnesium chloride), sometimes with the addition of base(sodium hydroxide or magnesium oxide) to raise pH of the watertreatment, and the phosphorus is recovered in the form of magnesiumammonium phosphate hexahydrate, i.e., struvite.

Such methods for phosphorus recovery have been proposed to be locatedeither before or after the anaerobic digestion of primary sludge andwaste activated sludge. FIG. 1, points 1 and 2, shows the location ofsuch P removal methods in a typical wastewater treatment plant. U.S.Pat. No. 7,182,872 (Barak, et al., 2007), as well as Jaffer et al.(2002), Britton et al. (2005), Bhuiyan et al. (2008) and Le Corre et al.(2007), considered the filtrate or the centrate of the anaerobicdigesters at sewage treatment plants, as in such as FIG. 1, point 2, asthe most promising spot for struvite formation, with the primarydeficiency—insufficient magnesium concentration in the filtrate—to beaddressed by addition of magnesium chloride (or magnesium-saturatedcation exchange resins, per Barak (2007) and pH to be adjusted upward ifneeded by addition of sodium hydroxide. Accordingly, Barak, et al.(2007) discloses a method and apparatus for removing phosphorus asstruvite from filtrate or centrate from anaerobic digester of a sewagetreatment plant by means of a negatively-charged compressed monolayer,self-assembled monolayer, or polymeric cation exchange membrane.Alternatively, U.S. Pat. No. 6,338,799 (Fukushima, et al.) discloses amethod for recovering phosphate from waste activated sludge in aphosphorus-releasing tank before anaerobic digestion, such as FIG. 1,point 1. The process includes treating sludge drawn from a watertreatment system at a sewage treatment plant in an anaerobic conditionto release polyphosphate accumulated in the sludge into solution, andrecovering phosphate in the solution using a seed crystal material. Thesystem for recovering phosphate from sludge includes aphosphorus-releasing means for treating sludge drawn from a watertreatment system at a sewage treatment plant in an anaerobic conditionto release phosphate into the bulk liquid, a dewatering and separatingmeans for separating the sludge containing the solution including thereleased phosphate into dewatering effluent and dewatered sludge, acalcium ion concentration-adjusting means for adjusting the calcium ionconcentration in the dewatering effluent, a means for adjusting the pHof the dewatering effluent to pH 7.5 to 9, and a crystallizing means forrecovering phosphate from the calcium ion concentration-adjusted,pH-adjusted effluent of dewatering apparatus.

In recent years, multi-phase anaerobic digesters have been introducedthat entail a sequence of organic acid digester, thermophilic digester,and mesophilic digester. FIG. 1 (bottom) shows such a multiphaseanaerobic digester. The purpose of such an arrangement is to optimizeenvironmental conditions for the several microbial processes involved inanaerobic digestion and thereby enhance methane production in thethermophilic phase and produce biosolids with reduced pathogen content.Key to the multi-phase process is the organic acid digester, whichproduces low molecular weight organic acids from digestiblecarbohydrates at mesophilic temperatures by processes of acidogenesisand acetogenesis, with a retention time of several days. The organicacid digest then passes to the thermophilic digester where, at highertemperature and higher pH, the microbial process of methanogenesisproduces methane in the form of biogas.

SUMMARY OF THE INVENTION

Described herein is a technology that captures up to 90% of thephosphorus in a usable form that can either be recycled to agriculturedirectly, or returned for reprocessing as a high-grade phosphate ore,which would have otherwise exited the sewage treatment plant asbiosolids. This technology manipulates the supernatant of themulti-phase anaerobic digestion process midstream, as opposed toreleasing a portion of the phosphate from the activated sludge beforeanaerobic digestion (Fukashima, et al.) or treating the filtrate orcentrate at the end of anaerobic digestion as described by others. Thistechnology takes advantage of the fact that phosphate is released fromthe biomass of polyphosphate accumulating organisms in the wasteactivated sludge and those phosphates are highly soluble in the mildlyacidic environment of the organic acid digester caused by thedecomposition of the digestible carbohydrates of the primary sludge. Theadoption of a three-phase anaerobic digestion system that entails anorganic acid digester, a thermophilic digester, and a mesophilicdigester presents an opportunity to remove phosphorus after it has beensolubilized during the process of acetogenesis and before it isre-precipitated in the solid phase in the thermophilic digesters, as inFIG. 1, point 3.

Such high levels of soluble phosphate in the organic digester areparticularly prone to controlled precipitation when pH values arebrought to near neutral values, especially when additional calcium (ormagnesium) is added to the mix. This may be accomplished by adding baseeither in the form of calcium carbonate and its calcined products,calcium oxide (lime) and calcium hydroxide (slaked lime); dolomite(calcium magnesium carbonate) and its calcined products; magnesite andits calcined products; or calcium-saturated weak-acid ion exchangeresin. Calcium-saturated resin may be prepared using either thebases—calcium carbonate, calcium oxide, or calcium hydroxide—oralternatively a fraction of the calcium bicarbonate-containing effluentwater, in which case a closed system has been created that does notrequire any further materials addition to the working wastewatertreatment plant.

For a more complete understanding of the phosphate recovery system andprocess of the present invention, reference is made to the followingdetailed description and accompanying drawings in which the presentlypreferred embodiments of the invention are shown by way of example. Asthe invention may be embodied in many forms without departing fromspirit of essential characteristics thereof, it is expressly understoodthat the drawings are for purposes of illustration and description only,and are not intended as a definition of the limits of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of the preferred embodiment of atemperature-phased anaerobic digestion facility of the phosphaterecovery system of the present invention, with the location ofphosphorus recovery labeled 3, compared with a conventional anaerobicdigester and placement of phosphorus recovery methods labeled 1 and 2.

FIG. 2 is a preferred embodiment of the organic acid digest treated withcalcium hydroxide for phosphate recovery by sedimentation against anup-welling flow in a fluidized bed reactor apparatus with particle sizesegregation.

FIG. 3 is a summary of the relationship between phosphorus remaining insolution after the various treatments—calcined carbonate minerals,sodium carbonate, and ion exchange resins as a function of pH conditionsinduced by the treatments.

FIG. 4 is a pH titration of digest with calcined calcite using thetemperature-phased anaerobic digestion process of the present invention.

FIG. 5A is a chemical analysis of acid digest after reaction withcalcined calcite (14 Mar. 2008) using the temperature-phased anaerobicdigestion process of the present invention.

FIG. 5B is a chemical analysis of acid digest after reaction withcalcined calcite (28 May 2008) using the temperature-phased anaerobicdigestion process of the present invention.

FIG. 6 discloses the relationship between phosphorus remaining insolution after the calcined calcite treatment—as a function of pHconditions induced by the treatment.

FIG. 7A depicts the chemical analysis of sediment from the acid digestafter reaction with calcined calcite (14 Mar. 2008) using thetemperature-phased anaerobic digestion process of the present invention.

FIG. 7B depicts the chemical analysis of sediment from the acid digestafter reaction with calcined calcite (28 May 2008) using thetemperature-phased anaerobic digestion process of the present invention.

FIG. 8 depicts the precipitated solids from organic acid digest (14 Mar.2008) upon addition of calcined calcite. A) top left, untreated digest;B) 0.62 g/L, pH 6.09; C) 1.08 g/L, pH 6.5; D) 1.55 g/L, pH 7.

FIG. 9 are X-ray diffraction patterns of precipitated solids from realacid digest sampled (28 May 2008) using the temperature-phased anaerobicdigestion process of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the preferred embodiment, an organic acid digester uses primarysludge and waste activated sludge as feedstocks and produces highconcentrations of volatile fatty acids. Typical suspended solids load of2 to 4%., expected pH is 4.6 to 5.1, soluble phosphorus concentrationsare 500 to 1200 mg/L, and retention times are 1.5 to 4 days.

In the first preferred embodiment, the organic acid digest is treatedfor phosphorus recovery before it is added to the thermophilic ormesophilic digesters. Such treatment may permit additional screening,floatation, sedimentation, filtration, or centrifugation of largeparticles in the digest before phosphate removal from the remainingliquid portion.

Phosphorus recovery from the organic acid digest is effected by raisingthe pH to near neutral value, not to exceed pH 7, and adding calcium bymeans of addition of calcium carbonate, calcium hydroxide, calciumoxide, calcium magnesium carbonate, calcium magnesium hydroxide, orcalcium magnesium oxide to the organic acid digest.

The resulting calcium phosphate product is to be collected bysedimentation (either static sedimentation in a settling tank orsedimentation in an upwelling flow as in a fluidized bed reactor, asdepicted in FIG. 2), filtration, or centrifugation. The processedorganic digest is sent to the thermophilic digester for methanogenesis.

In the second preferred embodiment, the organic acid digest either withor without particle removal, is applied to an ion exchange column loadedwith weak acid exchangers of the carboxylate, phosphonic,aminophosphonic, or other weak acid functional groups, loadedpredominantly with the calcium ion. Operation will be much like that ofstandard ion-exchange columns used for water softening. After the columnis spent, resulting calcium phosphate particles in the column are to beremoved by backwashing with tap water or plant effluent. The column willbe regenerated and returned to the calcium form by using calciumcarbonate, calcium hydroxide, calcium oxide, or either tap water orplant effluent containing soluble calcium bicarbonate.

In the third preferred embodiment, the organic acid digest is treatedfor phosphorus recovery and returned to the organic acid digester. As inthe first and second embodiments, additional screening, floatation,sedimentation, filtration, or centrifugation of large particles in thedigest is permitted before phosphate removal from the remaining liquidportion. Phosphate recovery will effected by either reaction withcalcium compounds as in the first embodiments or calcium-saturatedresins as in the second.

Experimental Example 1

In 2007, operators of the Nine Springs wastewater treatment plant,Madison Metropolitan Sewage District, Madison, Wis., replaced theirsingle stage, mesophilic anaerobic digestion system with a multi-phaseprocess and thereby separated the organic acid production from thebiogas production, as is increasingly practiced in the U.S. (see US-EPA,2006) and elsewhere. Large quantities of struvite were forming in theheat exchangers warming up the organic acid digest before entry into thethermophilic digesters and in the thermophilic digester itself, in theform of grit that sank to the bottom of the digester tank and restrictedthe effective volume of the digester.

On 8 Aug. 2007, 4-L samples were collected at the outflow of each of thedigesters and the filtrate (FIG. 1) at the Nine Springs WastewaterTreatment Plant, Temperature and pH were measured immediately.Characterization of the soluble components by collection of dialyzatewas begun immediately by placing dialysis bags (Spectra/Por 7 DialysisMembrane, MWCO: 50000; Spectrum Laboratories, Inc., Rancho Dominguez,Calif.) containing 25 mL of deionized water into the 4-L samples,shaking intermittently for 3 hrs. Chemical analysis of the anaerobicdigester dialyzates (see TABLE 1) showed significant chemicaldifferences among them. The organic acid digester had a highconcentration of volatile fatty acids, mostly acetic and propionicacids, and a pH of 4.69. This was in marked contrast to the slightlyalkaline pH of the dialyzates of the later thermophilic and mesophilicphases of the digestion process. Also of interest were the highconcentrations of phosphate, calcium, and magnesium in the organic aciddigester dialyzate, particularly in comparison to the later phases. Fromthe first stage of digestion (acid digest) until the centrifugationprocess, dissolved (dialyzable) phosphorus dropped from 18.5 mM to 6.2mM (161 ppm PO₄—P; see TABLE 2), calcium dropped from 5.3 mM to 0.61 mM(24.3 ppm, see TABLE 2) and magnesium dropped from 6.6 mM to 0.2 mM. Thedrop in phosphate, calcium, and magnesium concentrations between theorganic acid digester and the thermophilic digester suggests theprecipitation of calcium phosphates and struvite into the biosolidsduring the pH transition. Chemical modeling of the solutions (not shownhere) also suggests that this may be so.

Phosphorus concentrations in the GBT filtrate at the end of anaerobicdigestion process at the Nine Springs Plant are high compared to otherwaste water plants and published values (TABLE 2). However, a higherconcentration phosphorus source is to be found upstream in the organicacid digester in those plants with a multistage anaerobic digestionprocess. In light of this observation, the effectiveness of variousphosphorus removal treatments was determined based on the organic aciddigest.

TABLE 1 Analyses of Digester Samples at MMSD and Their Dialyzates.Organic GBT Acid Thermophilic Reactors Mesophilic Reactors FiltrateDigest: 7 6 5 4 3 2 1  EC, dS/m 4.48 6.33 6.30 5.95 6.10 6.32 6.38 7.69 pH 4.69 7.53 7.53 7.53 7.46 7.49 7.47 7.67  Temp, ° C. 38.6 51.8 52.252.3 39.4 38.6 39.2 34.1  Solids, % 5.10 2.60 2.50 2.70 2.30 2.20 2.400.20  Ca, % of solids 1.69 3.89 3.76 3.81 4.15 4.10 4.02 1.37  Mg, %   ″0.90 0.90 0.94 0.86 0.79 0.69 0.74 0.77  P, %    ″ 0.71 3.68 3.63 3.613.75 3.61 3.64 6.58 Dialyzate:  EC dS/m 5.82 8.23 8.19 7.74 7.93 8.228.30 8.39  Ca mM 5.3 0.4 0.5 0.5 0.5 0.6 0.6 0.6  Mg ″ 6.6 0.3 0.3 0.20.6 0.4 0.6 0.2  Na, ″ 9.4 10.3 10.3 10.2 10.0 10.0 10.0 10.6  K ″ 6.26.1 6.1 6.0 5.8 5.9 5.9 5.8  Fe²⁺ ″ 0.7 0.0 0.0 0.0 0.0 0.0 0.0 0.0 Mn²⁺ ″ 0.1 0.0 0.0 0.0 0.0 0.0 0.0 0.0  NH₄ ⁺ ″ 25.5 62.2 60.6 60.065.2 65.5 65.6 65.1  P ″ 18.5 6.2 6.3 6.3 5.8 5.7 6.0 5.2  Cl ″ 9.5 11.010.9 10.9 10.9 10.8 10.6 11.4  S ″ 1.0 0.2 0.2 0.2 0.2 0.1 0.3 0.2 Acetic acid ″ 22.9 0.4 0.4 0.4 0.3 0.3 0.3 0.2  Propionic acid ″ 21.40.0 0.0 0.0 0.0 0.0 0.0 0.0  N-Butyric acid ″ 4.9 0.1 0.1 0.1 0.1 0.10.1 0.1  N-Valeric acid ″ 2.4 0.0 0.0 0.0 0.0 0.0 0.0 0.0 IsoButyricacid ″ 1.2 0.1 0.1 0.1 0.1 0.1 0.1 0.1 IsoValeric acid ″ 0.8 0.0 0.0 0.00.0 0.0 0.0 0.0 SecValeric acid ″ 0.6 0.0 0.0 0.0 0.0 0.0 0.0 0.0

TABLE 2 Reported Soluble Phosphorus Concentrations in Centrate orFiltrate Following Anaerobic Digestion. P Ca Mg Location mg/L, (=ppm)Literature source British Columbia 71 — 28 A. Britton et al., 2005Slough, England 94.9 56 44 Jaffer et al., 2002 Milwaukee, GBT filtrate4.0 32.1 42.2 Barak et al., unpublished Madison, GBT filtrate 161 24.24.9 GBT filtrate, this study

To prepare synthetic organic acid digest approximating the compositionof the organic acid dialyzate sampled on 8 Aug. 2007, 1.284 g ofmagnesium acetate, 1.512 g of monobasic calcium phosphate, 0.816 g ofmonobasic potassium phosphate, 0.526 g of sodium chloride, and 1.927 gof ammonium acetate were added to a 1-L graduated flask and weredissolved in an acidic environment by adding acetic acid to a final pHof 4.67. Acetic acid was prepared by making up 10 g of glacial aceticacid to 500 mL with deionized water (335 mM) and then adding about 80 mLof the dilute acetic acid solution, sufficient to reach the target pH,to the other chemicals. The solution was then made up to 1 L withdeionized water and stirred continuously using a magnetic stirrer forabout 12 hours.

The synthetic organic acid dialyzate contains several overlapping pHbuffers: volatile fatty acids, e.g., acetic acid (pK 4.75); phosphate(pK₂ 7.2) and ammonium (pK 9.3). Initial experiments indicated that someof the carbonate minerals might not be reactive and they were thereforecalcined to oxides for comparative purposes.

High purity calcite (CaCO₃), dolomite (CaMg(CO₃)₂), and magnesite(MgCO₃) were ground to pass a 100-mesh sieve and calcined at 1000° C.for 8 hrs. The calcined materials, now in their oxide forms, were cooledin a dessicator and stored in sealed containers to avoid conversion tohydroxides and carbonates. Laboratory-grade sodium carbonate was used asa control in uncalcined form since it was readily soluble.

Calcium- and magnesium-saturated cation exchange resins of theweak-acid, carboxylic type, were prepared by neutralizing AmberliteIRC-50 (Mallinckrodt Chem. Works, St. Louis, Mo.; C.P., medium porosity,20-50 mesh size (wet), total exchange capacity: 3.5 meq/mL min.,supplied in W-form) with dilute ammonium hydroxide, rinsed with DIwater, left in calcium chloride or magnesium chloride brines solutionsovernight, and then rinsed with deionized water until the electricalconductivity was <100 dS/m. Excess water was removed with suction on aBuchner funnel and moist resins were stored for use.

Various weighed amounts of the calcined carbonate minerals and thecalcium- and magnesium-saturated resins were added to 40-mL aliquots ofthe synthetic organic acid dialyzate in test tubes and shaken gently for48 hrs. At 2, 4, 8, 24 and 48 hr, pH values were measured and shakingwas resumed. After 48 hrs, when it was determined that the chemicalreaction between the dialyzate and the added materials was completed,the tubes were centrifuged at 10,000×g for 20 min. Samples ofsupernatant were taken for analysis by ICP-OES and solids resuspended inthe remaining solution were transferred for light microscopy (Leica DML,Leica Microsystems Wetzlar GmbH, Nuhsbaum, Inc). The sizes of thesecrystalline particles were measured using Image J software (Version:1.39u, http://rsb.info.nih.gov/ij/, National Institutes of Health).Select samples were analyzed by x-ray diffraction at the S.W. BaileyX-ray Diffraction Laboratory (Dept. of Geology and Geophysics,UW-Madison) using a Scintag Pad V powder diffractometer with copper Kαradiation for identification of the minerals formed; experimentalconditions were 45 KeV, 40 mA, 0.02 degree step size, and 2 s dwell timeper step.

FIG. 3 is a graphical comparison between the phosphorus remaining insolution after the various treatments-calcined carbonate minerals,sodium carbonate, and ion exchange resins as a function of pH conditionsinduced by the treatments. The soluble phosphorus in the syntheticorganic acid solutions after treatment with the several calcinedcarbonate minerals, sodium carbonate, and metal-saturated resins showsthat phosphorus removal is foremost a matter of raising pH from theinitial value of about 4.8 to neutrality or above. Sodium carbonate,used here as a control, raises pH and precipitates phosphate as calciumphosphate, but not very effectively since it relies on the initialcalcium of the acid digest to do so; at pH 7, it has precipitated abouthalf of the initial P. By contrast, calcium oxide (calcined calcite)both raised pH by reacting with the acidity of the organic acids in thedigest and supplied additional calcium to promote calcium phosphateprecipitation; by pH 6.5, about 95% of the soluble phosphorus had beenprecipitated. Magnesium oxide (calcined magnesite) will form magnesiumammonium phosphate, struvite, at near-neutral and alkaline pH using theadded magnesium, but below neutrality the effect is largely like that ofsodium carbonate, i.e., precipitating calcium phosphate using theinitial calcium available in the acid digest; by pH 7, about 90% of thesoluble phosphorus has been precipitated, mostly as calcium phosphate.Calcined dolomite, providing an equivalent amount of CaO and MgO, splitsthe difference between the calcined calcite and calcined magnesitebehavior; it only begins to induce precipitation of magnesium phosphatessuch as struvite at near-neutral and alkaline pH. Taking the acid digestinto more alkaline conditions by adding either dolomite or magnesitefavors struvite formation but it may be recognized that phosphateprecipitation with calcium to form brushite (CaHPO₄.2H₂O), as opposed tostruvite (MgNH₄PO₄.6H₂O), is fundamentally advantageous to the removalof acidity since one mole of W is co-precipitated for each P removed. Asa locally-available material, the use of dolomite might be favored andits calcination could be accomplished with some of the energy from thecombustion of biogas. Although the work with calcined minerals wasaccomplished here using batch experiments, the use of either dolomite orcalcite might employ a fluidized bed reactor instead.

The use of weak-acid carboxylic ion exchangers to raise solution pH isfundamentally different from the use of oxides and carbonates, whichreact with the organic acids to produce H₂O and CO₂, respectively, plusthe appropriate organic (alkaline) anion. Instead, the weak-acid ionexchangers have an affinity for H₃O⁺ similar to that the organic acidsthemselves, with the addition of some additional electrostatic effectsdue to their polyvalent nature. Here, a calcium-saturated weak acidresin, though introducing no alkalinity into the organic acid digest,induced a pH rise from 4.8 to 5.9 and, in the process of doing so,precipitated 90% of the soluble phosphorus as calcium phosphates andthereby behaving very much like calcined calcium carbonate. Thistreatment with calcium-saturated resin bears further investigation. Theaddition of magnesium-saturated resin raised solution pH by the samemechanism but served as a sink for the initial calcium instead of asource of additional calcium for phosphate removal and was the onlytreatment tested that failed to precipitate at least some phosphorus.

Separate analyses of the weak-acid cation exchange resin in equilibriumwith the organic acid digest found that an exchangeable cation suite of52% H⁺, 26% Ca²⁺, 14% Mg²⁺, 4% NH₄ ⁺, 4% Fe²⁺, and 1% each Na⁺ and K⁺,with a total charge of 5.6 mmol(+)/g. If transferred to Madison citywater or MMSD effluent, the exchangeable H+ and other constituents wouldbe entirely replaced with calcium and magnesium by compulsive ionexchange, with the bicarbonate alkalinity of the water reacting with theacidity. Although the use of calcium-resin was here performed usingbatch experiments, the use of ion exchange columns for field testingwill likely be superior.

Experimental Example 2

The synthetic organic acid solutions used did not contain the microbialbiomass of the original digest and its effects on phosphate removal byprecipitation could not be predicted. Further, the work with thesynthetic acid digest used centrifugation at 10,000×g for 20 minutes toseparate supernatants and sediments, yet the observations using lightmicroscopy showed typical particles sizes in the sediment that couldhave been separated by centrifugal forces and treatment times morefriendly to the wastewater treatment plant than was used in thelaboratory. Therefore, the effectiveness of calcined calcite inprecipitating phosphate from the real organic acid digest from a workingwastewater treatment plant was tested and the phosphorus precipitatesidentified.

On 14 Mar. 2008 and 29 May 2008, samples were collected at the outflowof the organic acid digester (‘Digester #7’) into the thermophilicdigester at the Nine Springs wastewater treatment plant, MadisonMetropolitan Sewage District, Madison, Wis. Temperature and pH weremeasured immediately. Collection of dialyzate was begun immediately byplacing dialysis bags (Spectra/Por 7 Dialysis Membrane, MWCO: 50000;Spectrum Laboratories, Inc., Rancho Dominguez, Calif.) containing 25 mLof deionized water into the 4-L sample containers, shakingintermittently for 3 hrs, after which dialyzates were removed from thedialysis bags, electrical conductivity was measured in the dialyzatesand the dialyzates were analyzed for mineral constituents by inductivelycoupled plasma-optical emission spectroscopy (ICP-OES), volatile fattyacids by gas chromatography, and chloride by ion chromatography. Thespecific gravity and relative viscosity of the digest were measured at20° C. and 40° C. using a 50-mL pyknometer and a lab-built Ostwaldviscometer with a 25-mL reservoir, respectively.

Calcined calcite was prepared by heating powdered calcitic limestone at1000° C. for 4 hr. Calcined calcite was added to 40-mL aliquots of theorganic acid digests, with the highest rate intended to bring the pH ofthe digest close to neutral or above. The samples were shaken for 48 hrand then centrifuged at 179 relative centrifugal force (1000 rpm) for 4min, of which 2 min was acceleration to the desired speed and 2 min atthe desired speed. The centrifugal force and duration of centrifugationwere chosen based on digest density and viscosity that would, followingStokes Law:

$V_{s} = {\frac{2}{9}\frac{\left( {\rho_{p} - \rho_{f}} \right)}{\mu}g\mspace{11mu} R^{2}}$

where V_(s) is terminal settling velocity;

R is particle radius;

g is centrifugal or gravitational force, here calculated for acentrifuge where relative centrifugal force=11.17 r (ω/1000)², with a16-cm centrifuge radius, and ω is angular velocity in revolutions permin;

ρ_(p) is particle density;

ρ_(f) is density of the fluid, here 1.000 at 40° C. for the organic aciddigest;

μ is viscosity of the fluid, here a relative viscosity of 1.13 at 40°C., that would cause a brushite particle of 0.1 mm radius and density of2500 kg/m³ to settle 9 cm and thereby clear the supernatant in the 40-mLcentrifuge tubes used.

The 0.1 mm dimension was chosen based on preliminary experiments withsynthetic dialyzates wherein the size of the brushite crystals variedfrom about a 0.02 mm to 1.5 mm based on light microscopy and imageanalysis using ImageJ (v. 1.39u, http://rsb.info.nih.gov/ij/, NationalInstitutes of Health).

After centrifugation, the supernatant was separated from the sediment byaspiration, with subsamples dried at 105° C. to determine percent totalsolids, and then ashed at 550° C. to remove organic matter. The ashedsupernatant was dissolved in 1% HNO₃, filtered, and analyzed by ICP-OES.Sediments after centrifugation were dried at 60° C. Subsamples wereinspected by light microscopy and by x-ray diffraction using a ScintagPad V powder diffractometer with copper Kα radiation for identificationof the minerals formed. Experimental conditions were 45 KeV, 40 mA, 0.02degree step size, and 2 s dwell time per step.

The organic acid digests were acidic, pH 4.67 and 5.00 on the twosampling dates, and solutions contained considerable concentrations ofCa, Mg, and phosphorus (see TABLE 3) compared to the digests ofthermophilic digesters. Compared to the results of the earlier samplingdate, 8 Aug. 2007, and the later date, 28 May 2008, the composition ofthe 14 Mar. 2008 was unusually high in Fe and relatively low in P.Discussion with the operators of the Nine Springs plant revealed thatduring that time the operators were adding considerable amounts offerric chloride to the organic acid digester in an attempt to reduce thepH and reduce foaming. This treatment had the undesirable consequence ofcausing the formation of vivianite, Fe₃(PO₄)₂. 8H₂O, in the heatexchangers and was discontinued, whereupon the level of soluble Fedropped and level of soluble phosphorus rose by the time of the 28 May2008 sampling, which resembled the August 2007 results.

The rates of calcined calcite to neutralize the organic acids in thedigests and raising pH to about 7 were 1.55 and 1.1 g per L acid digestsfor 14 March and 28 May, respectively (see FIG. 4). Analysis of thetotal solids in the supernatant (see TABLE 3) shows that the addition ofcalcined calcite did not cause general precipitation of the suspendedsolids. Chemical analyses of the supernatants demonstrate thatphosphorus in the organic acid digest was removed from solution as pHwas raised with calcined calcite, dropping from about 9 mM in theuntreated digest to 3 mM with 1.55 g/L calcined calcite in the earliersampling date (see FIG. 5A), and from about 17 mM to 2 mM with 1.1 g/Lcalcined calcite on the latter sampling date (see FIG. 5B). Thesetreatments did not alter the concentrations of soluble iron, sodium, orpotassium; soluble magnesium increased slightly with addition ofcalcined calcite, perhaps due to small impurities of magnesium in thesource rock. Also, concentrations of calcium remaining in the digestsdid not increase in proportion to the amount of calcined calcite addeduntil after the phosphorus concentrations had been reduced. On raisingthe pH above 7, there is a decline in magnesium concentration in thesupernatant, perhaps indicative of precipitation of magnesium phosphatesand magnesium ammonium phosphates.

Referring now to FIG. 6, when the concentration of phosphorus in thesupernatant is considered as a function of pH, it becomes clear thatthere is a portion of the initial phosphate that is not precipitatedunder these conditions. This may be phosphorus in the suspended biomassitself since the calcined calcite treatment did not clear the digestcompletely of its contents (TABLE 4). Interestingly, the digest with thehigher initial phosphorus content, that of 28 May 2008, had the greateramount of phosphorus removed upon being neutralized and the remainingphosphorus was brought to a lower concentration after neutralization;the amount of calcined calcite required for neutralization was less thanthat required for the 14 Mar. 2008 sample.

Chemical analysis of the sediments, as depicted in FIG. 7A, demonstratethat the 14 Mar. 2008 organic acid digest had a relatively high contentof iron and phosphorus that could be removed by centrifugation,independent of the treatment. The 28 May 2008 organic acid digest, whichbetter represented normal operational conditions, had a high percentageof phosphorus in the sediments, as high as 13% by weight (see FIG. 7B).The major auxiliary element was calcium, suggesting that calciumphosphates are the major constituent.

The precipitated solids were examined under the microscope to examinethe shape, size and the formation of crystals. The acid digest samplewith the calcined calcite sufficient to raise the pH to 7 had thegreatest number of crystals, as shown in FIG. 8, and precipitation ofthe crystals was seen to increase on increasing the amount of calcinedcalcite added. The crystals formed in the acid digest were similar inappearance to those formed in the earlier experiments with the syntheticacid digest. They were platy in structure and were observed to benucleated on the surface of calcined calcite particles. The size of thecrystals varied from about 0.02 mm to 1.5 mm, similar to the previousstudy with synthetic acid digest using calcined carbonates.

Upon examination by X-ray diffraction, it was observed (FIG. 9) that thepredominant mineral that had formed in the real acid digests wasbrushite, particularly when the digest was still acid. For the firstsampling date, brushite and small amounts of magnesium phosphate wereidentified at pH values ranging from 6.1-7. For the second samplingdate, at pH values ranging from 6.2-7, along with the mineral brushite,a small amount of gypsum was also identified; at yet higher pH(8.6-10.8), substantial amount of calcite mineral phases wereidentified. For both the sampling dates, untreated digest did not showsigns of any mineral phase identifiable by x-ray. Given the relativelylow peak counts, it is possible that a significant portion of theprecipitated solids did not give strong x-ray diffraction, and maytherefore be regarded as ‘amorphous’ for the purpose of x-rayidentification. Amorphous calcium phosphate is expected to have the sameutility as crystalline calcium phosphate in the form of brushite, bothfor agronomic purposes and as a phosphorus ore for industrial purposes.

Under working conditions, the phosphate precipitates can be recovered bysedimentation (either static or upwelling flow), centrifugation, orfiltration (such as a gravity belt thickener). It may be envisioned thatthe organic digest may be pretreated by sedimentation, centrifugation,or filtration to remove entrained grit and non-phosphate particles sothat such particles are not collected together with the phosphateprecipitates produced by raising the pH and adding calcium andmagnesium.

From these results it can be seen that raising the pH and adding calciumthrough addition of calcined calcite to the organic acid digest removephosphorus from the anaerobic digester, thereby reducing its nuisancevalue and recovering it for recycling as a plant nutrient.

TABLE 3 Test Results - Chemical Composition of Digests and Dialyzates ofDigests from MMSD. 08 Aug 2007 14 Mar 2008 28 May 2008 Organic ThermoOrganic Thermo Organic Thermo Acid -philic Acid -philic Acid -philicDigest:  pH 4.69 7.53 4.67 7.01 5.0 7.52  Temp, ° C. 43.6 52 28.7 43.631.2 49.6 Dialyzate:  EC, dS/m 5.82 8.2 5.92 7.17 3.56 6.47  P mg/L572.23 159.98 275.4 47.78 440 193  K ″ 241.37 237.88 182.2 191.7 229 246 Ca ″ 213.4 17.68 310.0 91.3 203 28  Mg ″ 159.98 6.21 119.5 46.9 138 20 S ″ 30.9 5.9 22.9 2.6 9.7 4.8  Mn²⁺ ″ 5.07 0.02 5.44 0.10 7.0 7.0  Fe²⁺″ 38.7 0.9 286.3 1.05 9.8 9.8  Na ″ 217.14 237.2 243.9 261.6 236 238 NH₄—N ″ 357 871 334.4 757.6 218 620  CL⁻ ″ 338 389.9 812.2 962.8 362355  Acetic acid ″ 1372 21.7 1397 185 1519 8.46  Propionic acid ″ 15890.5 929 140 718 1.67  IsoButyric acid ″ 103 5 72.3 7.1 68 ND* IsoValeric acid ″ 79.8 4.9 51.6 6.51 57.6 ND*  N-Butyric acid ″ 436 5295 34.2 406 ND*  N-Valeric acid ″ 242.9 4.9 174 22.6 141 ND* SecValeric acid ″ 60.2 4.9 41.7 5.05 38.5 ND* *Not Determined

TABLE 4 Percentage of Total Solids in Organic Acid Digests afterAddition of Calcined Calcite. Calcined calcite added, g/L acid digestSampling Date 0 0.62 1.08 1.55 2.0 2.5 - - - Percent total solids - - -14 Mar 2008 0.50 0.62 0.78 0.675 28 May 2008 0.43 0.44 0.48 0.47 0.580.55

LIST OF REFERENCES

-   Barak, P., M. E. Tabanpour, M. Avila-Segura and J. M. Meyer. 2007.    Struvite Crystallization. U.S. Pat. No. 7,182,872; rights assigned    to WARF.-   Bhuiyan, M. I. H., D. S. Mavinic and F. A. Koch. 2008. Phosphorus    recovery from wastewater through struvite formation in fluidized bed    reactors. Water Sci. Technol. 57(2):175-181.-   Britton, A., F. A. Koch, D. S. Mavinic, A. Adnan, W. K. Oldham    and B. Udala. 2005. Pilot-scale struvite recovery from anaerobic    digester supernatant at an enhanced biological phosphorus removal    wastewater treatment plant. J. Environ. Engin. Sci. 4:265-277.-   Durrant, A. E., M. D. Scrimshaw, I. Stratful and J. N. Lester. 1999.    Review of the feasibility of recovering phosphate from wastewater    for use as a raw material by the phosphate industry. Environ.    Technol. 20:749-758.-   Fukushima, Y., T. Matsumoto, K. Kawabata, and K. Moriyama. 2002.    Method for recovering phosphate from sludge and system therefore.    U.S. Pat. No. 6,338,799 B1. Jan. 15, 2002.-   Hirasawa, I., K. Okada, Y. Hoshino, K. Shimada and M. Nagauchi.    1981b. Studies on phosphorus removal by contact crystallization from    sewage (2). J. Japan Sewage Works Assoc. 18(204):38-45.-   Hirasawa, I., K. Okada, Y. Hoshino, K. Shimada and M. Nagauchi.    1981c. Studies on phosphorus removal by contact crystallization from    sewage (3). J. Japan Sewage Works Assoc. 18(205):11-19.-   Hirasawa, I., K. Okada, Y. Hoshino, K. Shimada and M. Nagauchi.    1981a. Studies on phosphorus removal by contact crystallization from    sewage (1). J. Japan Sewage Works Assoc. 18(203): 12-21.

Throughout this application, various patents, and publications arereferenced. The disclosures of these documents in their entireties arehereby incorporated by reference into this specification in order tomore fully describe the state of the art to which this inventionpertains.

It is evident that many alternatives, modifications, and variations ofthe phosphate recovery system and process of the present invention willbe apparent to those skilled in the art in light of the disclosureherein. It is intended that the metes and bounds of the presentinvention be determined by the appended claims rather than by thelanguage of the above specification, and that all such alternatives,modifications, and variations which form a conjointly cooperativeequivalent are intended to be included within the spirit and scope ofthese claims.

What is claimed is:
 1. A process for removing phosphates from wastewatercontaining solubilized phosphates, the process comprising the steps of:raising the pH of the wastewater to a pH of less than 8 by addingcalcium containing compounds to the wastewater, whereby solubilizedphosphates are precipitated and the precipitated phosphates comprisebrushite; and removing the precipitated phosphates from the wastewater.2. The process of claim 1, wherein the calcium containing compoundscomprise calcium oxide or calcium hydroxide.
 3. The process of claim 1,wherein the calcium containing compounds consist of calcium oxide orcalcium hydroxide.
 4. The process of claim 1, wherein the pH is raisedto a value of no greater than
 7. 5. The process of claim 1, wherein thepH is raised to a value in the range from 6.1 to
 7. 6. The process ofclaim 1, wherein brushite is the predominant mineral in the precipitatedphosphates.
 7. The process of claim 1, further comprising removingnon-phosphate particles from the wastewater prior to raising the pH ofthe wastewater.
 8. A process for removing phosphates from wastewatercontaining solubilized phosphates, the process comprising the steps of:raising the pH of the wastewater to a value in the range from 6.1 to 7by adding calcium oxide or calcium hydroxide to the wastewater, wherebysolubilized phosphates are precipitated and further wherein brushite isthe predominant mineral in the precipitated phosphates; and removing theprecipitated phosphates from the wastewater.
 9. The process of claim 8,further comprising removing non-phosphate particles from the wastewaterprior to raising the pH of the wastewater.